camera lucida and camera obscura

The principle of the camera obscura was known to Aristotle;
light admitted through a small hole into a darkened chamber projects a real
image of the scene outside on the opposite wall. Later versions have used
lenses and mirrors
to give an evenly-illuminated horizontal image.

The camera lucida, invented in 1807 by William Wollaston,
employs a four sided prism to allow the artist
to see a virtual image of an object in the plane of the paper on which the
image is to be copied. It used to be of particular use in enlarging or reducing
artwork and in drawing from the microscope.